Cultured keratinocytes isolated from skin of young and old individuals will be used as a model system to study the phenomenon of aging and cellular senescence. We will compare growth and maturation of the respective cultures and in particular will examine their DNA repair capabilities. Irradiation with ultraviolet light will be used to produce repairable damage (principally pyrimidine dimmer) in the cellular DNA. Loss of dimers from DNA and repair will be assessed by sensitivity to the phage T4 endonuclease V (specific for dimer-containing DNA strands) and by the incorporation of radioactively labeled 5-bromodeoxyuridine into the DNA as measure of repair replication. Since daily exposure of humans to sunlight contributes to the manifestations of aging in exposed skin we will mimic this treatment by periodic conditions of our cultures with sublethal, low dose ultraviolet exposure. The effect of a larger, challenge dose of ultraviolet light on DNA replication and cell growth will then be assessed. We will look for confirming evidence of inducible recovery mechanisms and the possibility that the induced response differs in the keratinocytes from young and old donors. (Our previous work using survival of UV irradiated simian virus 40 as test probe has provided evidence for a recovery enhancement when host mammalian cells are treated with low levels of UV or chemical carcinogens). We will also compare the growth profile and DNA repair activity in cultured epidermal keratinocytes from involved and uninvolved skin of patients with psoriasis. These results will be correlated with those from normal individuals of corresponding ages.